In present day hydrocarbon exploration many tools are used, from initial seismic exploration to logging equipment when a well has been drilled. One such piece of logging equipment is the neutron logging tool.
Neutron logs are used for determination of porous foundations and their porosity. They respond primarily to the amount of hydrogen present in the formation. Thus, in clean formations, that is where there has not been invasion by drawing mud, whose pores are filled with water or oil, the neutron log reflects the amount of liquid filled porosity.
Gas zones can be identified by comparing the neutron log with other porosity log or a core analysis. A combination of the neutron log with one or two other porosity logs yields even more accurate porosity values and lithology identification, including the evaluation of shale content.
Neutrons are electrically neutral particles, each having a mass almost identical to the mass of a hydrogen atom. High energy neutrons are continuously emitted from a radioactive source which is mounted in the Sonde. These neutrons collide with nuclei of the formation material in what is termed an elastic "billiard ball" type of collision. With each collision a neutron loses some of its energy.
The amount of energy lost per collision depends on the relative mass of the nucleus with which the neutron collides. The greatest energy loss occurs when the neutron strikes a nucleus of practically equal mass such as a hydrogen nucleus. Collisions with heavier nuclei do not cause as great an energy loss. The energy loss of neutrons from the neutron logging tool depends primarily on the amount of hydrogen in the formation.
When the hydrogen concentration of the material surrounding the neutron source is large, most of the neutrons have a great energy loss within a short distance of the source. However, if the hydrogen concentration is small, the neutrons travel farther from the source before incurring a great energy loss. Accordingly, the counting rate of the detector increases for decreased hydrogen concentration and decreases for the absence of hydrogen.
A neutron detector generally comprises a gas filled detector used in conjunction with a charged coupled amplifier, post amplifier and a discriminator.
The neutron detector generally comprises a charged coupled amplifier used in conjunction with a post amplifier and a discriminator. Normally the charge amplifier must be separated physically from three to four feet from the post amplifier and discriminator. Unfortunately, this type of system used to discriminate the analog pulses is prone to temperature drifts. Furthermore, shielding the connections between the charge amplifier and the post amplifier becomes an increasing problem as the distance between the two is increased.